Concept explainers
Consider the spacecraft heat rejection scheme of Problem 13.27, but under conditions for which surfaces 1 and 2 may not be approximated as blackbodies.
(a) For isothermal surfaces of temperature
(b) Explore the effect of the emissivity on the rate of heat rejection, and contrast your results with those for emission exclusively from the base of the section.
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Fundamentals of Heat and Mass Transfer
- 11.31 A large slab of steel 0.1 m thick contains a 0.1 -m-di- ameter circular hole whose axis is normal to the surface. Considering the sides of the hole to be black, specify the rate of radiative heat loss from the hole. The plate is at 811 K, and the surroundings are at 300 K.arrow_forwardDetermine the total average hemispherical emissivity and the emissive power of a surface that has a spectral hemispherical emissivity of 0.8 at wavelengths less than 1.5m, 0.6 at wavelengths from 1.5to2.5m, and 0.4 at wavelengths longer than 2.5m. The surface temperature is 1111 K.arrow_forward11.68 Two infinitely large, black, plane surfaces are 0.3 m apart, and the space between them is filled by an isothermal gas mixture at 811 K and atmospheric pressure. The gas mixture consists of by volume. If one of the surfaces is maintained at 278 K and the other at 1390 K, calculate (a) the effective emissivity of the gas at its temperature, (b) the effective absorptivity of the gas to radiation from the 1390 K surface, (c) the effective absorptivity of the gas to radiation from the 278 K surface, and (d) the net rate of heat transfer to the gas per square meter of surface area.arrow_forward
- 11.41 Determine the steady-state temperatures of two radiation shields placed in the evacuated space between two infinite planes at temperatures of 555 K and 278 K. The emissivity of all surfaces is 0.8.arrow_forwardA long wire 0.7 mm in diameter with an emissivity of 0.9 is placed in a large quiescent air space at 270 K. If the wire is at 800 K, calculate the net rate of heat loss. Discuss your assumptions.arrow_forward1.26 Repeat Problem 1.25 but assume that the surface of the storage vessel has an absorbance (equal to the emittance) of 0.1. Then determine the rate of evaporation of the liquid oxygen in kilograms per second and pounds per hour, assuming that convection can be neglected. The heat of vaporization of oxygen at –183°C is .arrow_forward
- Determine the rate of radiant heat emission in watts per square meter from a blackbody at (a) 15C, (b) 600C, and (c) 5700C.arrow_forwardA tube carries hot water across a factory in a tube with outer diameter Do = 20 mm. The tube surface is black, and the surroundings are at 20°C. You may neglect convection during your analysis. b) To reduce the rate of heat loss from the pipe, you decide to surround the pipe in a radiation shield. The shield material you have has inner and outer emissivities of E2,i = 0.01 and 2,0 = 0.1, respectively. Calculate the rate of heat transfer out of the tube, per unit length of tube, if the tube surface remains at 450°C and the radiation shield has a diameter of 60 mm. Shield, D₂ = 60 mm 2,0 E2,i Heated tube, D₁₂ = 20 mm Evacuated Page 3 of 4arrow_forwardA long, thin-walled horizontal tube 10.0cm in diameter with an emissivity of 0.80 is maintained at 120.0°C by the passage of pressurized steam inside the tube. To reduce radiation heat transfer losses, a round, tubular radiation shield with an emissivity of 0.10 is installed surrounding the heated tube, with a gap of 1.00cm between the outside of the tube and the radiation shield. At steady-state operating conditions, the temperature of the radiation shield is measured to be 35.0°C. Calculate the net radiation heat transfer rate from the tube to the radiation shield per unit length of pipe, ignoring any convection heat transfer in the annular space between the tube and the radiation shield.arrow_forward
- A long, thin-walled horizontal tube 10.0cm in diameter with an emissivity of 0.80 is maintained at 120.0°C by the passage of pressurized steam inside the tube. To reduce radiation heat transfer losses, a round, tubular radiation shield with an emissivity of 0.10 is installed surrounding the heated tube, with a gap of 1.00cm between the outside of the tube and the radiation shield. At steady-state operating conditions, the temperature of the radiation shield is measured to be 35.0°C. Calculate the total heat transfer rate per meter length assuming that the gap between the pipe and heat shield is now filled with air at a pressure of 1.00 atm.arrow_forward1. A small gray sphere, with an emissivity coefficient of 0.5 and a surface temperature of 537°C, is located in a black body wrap with a temperature of 35°C. For this system, calculate the net rate of heat transfer per unit of surface area of the sphere. 2. Gaseous oxygen is maintained at pressures of 2 atm and 1 atm on the opposite sides of a rubber membrane, which has a thickness of 0.5 mm, and the entire system is at 25°C. What is the diffusive mass flow of gas through the membrane? DAB=0.21x10^-9 m^2/s; O = 16 g/mol 3. Pure oxygen gas at 2 bar and 25°C is flowing through a rubber hose of 10 m long, with 40 mm internal diameter and 2 mm wall thickness. The external surface is exposed to an air stream in which the partial pressure of the gas is 0.1 bar. The diffusivity and solubility of the gas in the hose material are 0.21x10^-9 m^2/s and 3.12x10^-3 kmol/m^3.bar. respectively. Determine the mass rate at which the gas leaks out of the hose. 4. Consider the diffusion of gaseous…arrow_forwardAn opaque, diffuse, gray, square (200 mm x 200 mm) plate with an emissivity of 0.8 is placed over the opening of a furnace (L = 200 mm) and the plate temperature is known to be 400 K at a certain instant. The bottom of the furnace, having the same dimensions as the plate, is black and operates at 1040 K. The sidewalls of the furnace are well insulated. The top of the plate is exposed to ambient air with a convection coefficient of 25 W/m².K and to large surroundings. The air and surroundings are each at 300 K. Air 9₁ = To h = W -Plate (a) Evaluate the net radiative heat transfer to the bottom surface of the plate, in W. T SUT -Insulated sidewalls -Furnace bottom O (b) If the plate has mass and specific heat of 2 kg and 900 J/kg-K, respectively, what will be the change in temperature of the plate with time, dTp/dt, in K/s? Assume convection to the bottom surface of the plate to be negligible. dT₁ = K/s dtarrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning